Petroleum drilling and production operations require large quantities of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” may be performed by several techniques that may require the transmission of large quantities of information through the borehole.
In conventional oil well wireline logging, a tool string containing a number of tool assemblies that may each include sensors, actuators, memory, and processors is lowered into the borehole after some or all of the well has been drilled. The tool string is used to determine characteristics of the earth formations traversed by the borehole. The upper end of the tool string is attached to a multi-conductor wireline cable that suspends the tool string in the borehole. A downhole communication bus coupled to the wireline cable travels through the length of the tool string, connecting the tool assemblies together. Power may be transmitted to the sensors and instrumentation in each of the tool assemblies through the single conductive wireline. Instrumentation in the tool string may communicate information to the surface by using a modem to transmit signals through the downhole communication bus and wireline cable.
Another technique for the collection of information relating to downhole conditions and formation properties is known as “logging-while-drilling” or “LWD.” LWD techniques include the collection of data during the process of drilling the borehole. Collecting and processing data during the drilling process eliminates the necessity of removing the drilling assembly to insert a wireline logging tool string. It consequently allows the drill operator to make accurate modifications or corrections as needed to optimize performance while minimizing down time.
Once information from the borehole is gathered, it may be necessary to communicate it to other tool assemblies in the tool string and to the surface for use and analysis by the tool operator. Thus, for example, in an LWD system, the direction and inclination of the drill bit and downhole motor may be needed by tool assemblies in the tool string so that the tool string may be steered in the correct direction. Additionally, information may be required by the tool operator concerning the nature of the strata being drilled, such as the formation's resistivity, velocity, porosity, density and its measure of gamma radiation. The tool operator may want to know other downhole parameters, such as the temperature and the pressure at the base of the borehole.
Collection of information may be done by a number of separate tool assemblies that form the tool string. As mentioned above, the tool assemblies may have to communicate with each other to transmit information collected by one tool assembly to another tool assembly. Thus, a tool assembly that connects to the drill bit and downhole motor may frequently need to communicate the direction and inclination information to another tool assembly that adjusts the steering of the tool string. In some scenarios, a tool assembly may need to transmit information to all tool assemblies in the tool string, thus requiring a “global broadcast” capability. Finally, each tool assembly in the tool string may gather information concerning the nature of the earth strata being drilled and transmit this information through other tool assemblies to a central recorder located on-board the tool string that records the information. Due to power and technology limitations, existing LWD tool strings with a single wire communication bus have limited throughput.
Various techniques to increase the communication bandwidth and the speed of information transfer between tool assemblies in a tool string have been suggested. These techniques include increasing the data transmission rate over the single wire connecting tool assemblies or increasing the number of wires connecting tool assemblies to implement parallel information transfers. However, such techniques increase power consumption, are not compatible with existing tool assemblies, and require substantial redesign and modification of these assemblies. Thus, there is a continuing need for a high speed, high bandwidth communication system that allows tool assemblies to transfer large quantities of data to each other which is compatible with existing tool assemblies.